1. Field of the Invention
The invention relates to amplifier circuits. More particularly, the invention relates to transimpedance amplifiers with improved gain-bandwidth products.
2. Prior Art
As described in greater detail below, the open loop gain of transimpedance amplifiers is reduced with lower operating voltages. This reduction in open loop gain results in lower gain-bandwidth product when other factors remain constant. As electronic systems move to lower operating voltages or power management or other purposes, the performance of transimpedance amplifiers included in the system decreases. The decreased performance of the transimpedance amplifier results in overall system performance decreases.
FIG. 1 is a circuit diagram of a transimpedance amplifier coupled to a photodiode. The circuit of FIG. 1 converts light modulated current in the photodiode to an output voltage (V.sub.OUT). The circuit is useful, for example, for receiving data transmitted via fiber optic lines.
In general, the transimpedance gain of the circuit is given by ##EQU1## where R.sub.RT is the resistance of resistor RT and AV is the absolute open loop voltage gain of the amplifier portion of the circuit. In the circuit of FIG. 1, the open loop voltage gain is the voltage gain from the base of Q1 to the emitter of Q3 with RT removed. Because thermal noise current generated by resistor RT is a dominant noise source and limits sensitivity of the transimpedance amplifier, it is advantageous to make resistor RT as large as possible to reduce the corresponding noise current contribution. Current noise in RT is given by: ##EQU2## where k is Boltzmann's constant, T is temperature, and .DELTA.f is the frequency band. Thus, increasing the resistance of RT decreases current noise.
High open loop gain allows the resistor RT to be made larger, and therefore less noisy, without sacrificing bandwidth because the resistance of resistor RT as seen from the input of the transimpedance amplifier is divided by 1+AV as a result of feedback. Because the bandwidth of the transimpedance amplifier is inversely proportional to the product of the input capacitance and the input resistance, the reduction of R.sub.RT by a factor of 1+AV allows R.sub.RT to be very large while maintaining a high bandwidth for the transimpedance amplifier.
The open loop gain of the circuit of FIG. 1 is approximately ##EQU3## where gm.sub.Q1 is the transconductance of transistor Q1 and R.sub.RL is the resistance of resistor RL (e.g., load resistance). Given ##EQU4## where I.sub.C.sbsb.Q1 is the collector current of transistor Q1 and V.sub.T is the thermal voltage, kT/q, and ##EQU5## when the current in photodiode is zero, where V.sub.CC is the supply voltage and V.sub.BE is the voltage between the base and the emitter of transistor Q1 or Q3, the open loop gain of the transimpedance amplifier can be expressed as: ##EQU6## Thus, the open loop gain of the transimpedance amplifier is largely a function of the supply voltage. Transistor Q3 forms an emitter follower that allows the output of the circuit to have a relatively low impedance. Current source I1 biases transistor Q3 to operate for the full range of output voltages.
In an implementation having a 5 Volt supply voltage and V.sub.BE .apprxeq.0.8 at room temperature, ##EQU7## With a 3 Volt supply voltage, however, AV drops to about 54V/V. This gain reduction results in either lower bandwidth if the circuit is not changed or more noise if R.sub.RT is decreased to improve bandwidth.